The invention relates to an arrangement for pneumatic false-twist spinning having a false-twisting zone which is situated between a drafting unit and a withdrawal device and which comprises an intake nozzle following the drafting unit and a false-twisting nozzle in front of the withdrawal device, between which a suction roller is arranged which is used as a yarn guide and is driven to perform a movement in the travelling direction of the yarn.
In the case of a known arrangement of the initially mentioned type (see German Patent Document DE-A 36 31 400, FIG. 25), a suction roller is arranged between an intake nozzle and a false-twisting nozzle by means of which it is to be at least partially prevented that the twist provided to the sliver by the false-twisting nozzle moves back. This has the purpose of improving the spreading-away of the fiber ends. The circumferential speed of the drivable suction roller corresponds to the delivery speed of the drafting unit.
It is an object of the invention to develop an arrangement of the initially mentioned type in such a manner that a yarn is obtained that has an increased strength and a more uniform appearance.
This object is achieved according to the invention in that the suction roller is driven to rotate at a circumferential speed which is higher than the delivery speed of the drafting unit and the withdrawal speed of the withdrawal device.
Because of this construction, the edge fibers are taken along by the suction roller at an increased speed in comparison to the core, the speed of which is determined by the delivery speed of the drafting unit and the withdrawal speed of the withdrawal device. These edge fibers are therefore wound in a defined form around the core which, because of the false twist provided by the false-twisting nozzle, rotates around its axis on the circumference of the suction roller. After the false twist has opened up, these edge fibers remain wound around the core of the yarn in the defined form so that a firm and uniform yarn is obtained.
In a further development of the invention, it is provided that the suction roller comprises a suction insert having a suction slot which extends in the circumferential direction of the suction roller from an outlet area of the intake nozzle into an inlet area of the false-twisting nozzle. This determines the area in which the ends of the edge fibers in a defined form are wound around the yarn core which rotates around its axis because of the false twist. In this case, it is advantageous for the outlet of the intake nozzle, which with its yarn duct is aligned essentially tangentially with respect to the circumference of the suction roller, to be arranged at a distance to the circumference of the suction roller, and for the suction slot of the suction roller to extend against its rotating direction at least to an area opposite the outlet of the intake nozzle. In the area of the sliver, which is freely guided between the outlet of the intake nozzle and the circumference of the suction roller, fiber ends can be spread away very uniformly by being sucked toward the circumference of the suction roller and thus obtaining a component of motion which differs from the motion of the yarn core directed tangentially to the suction roller.
In this case, it is also expedient if it is provided in a further development of the invention that the area of the outlet of the intake nozzle, on the side facing the suction roller, is open diagonally to the yarn travelling direction. It is achieved in this manner that the flow-off direction of the air quantity flowing out of the yarn duct of the intake nozzle deviates from the yarn travelling direction so that the spreading-away of the edge fibers is supported further.
In a further development of the invention, it is provided that the output of the suction insert is coordinated such with the air quantity flowing out of the intake nozzle that this air quantity is sucked at least approximately completely into the suction slot. This results in a very defined air flow in the outlet area of the intake nozzle which further supports the spreading-away of fiber ends.
In a further development of the invention, it is provided that the intake width of the suction nozzle, in the axial direction of the pair of delivery rollers of the drafting unit, is wider than the width of a covering of a pressure roller. In this case, it is particularly advantageous for the width of the coating of the pressure roller of the pair of delivery rollers to correspond essentially to the width of the drawn sliver. As a result, it is prevented that the air flow entrained by the rollers of the pair of delivery rollers of the drafting unit disturbs the fiber arrangement on the inlet side of the pair of delivery rollers. On the contrary, air currents which promote the orientation of the fiber ends are taken in through the area of the nip line of the pair of delivery rollers.
In a further development of the invention, it is provided that the air flow rate in the intake nozzle is higher than the delivery speed of the drafting unit. As a result, it is achieved that the fiber ends are, in addition, aligned toward the front in the travelling direction of the yarn. Also, fibers, which are so-called floating fibers and which, for example, are no longer clamped into the pair of delivery rollers and have not yet reached the suction roller, are guided in a more controlled manner. This development is advantageous particularly if a sliver is processed which does not have very long fibers, such as a short-staple or a medium-staple sliver.